问题
选择题
Most birds find it safe to sleep in the trees, but ________they have eggs or young chicks, they don’t use a nest.
A．why
B．how
C．unless
D．where
答案
C。
由they have eggs or young chicks与they don’t use a nest之间的逻辑关系可以推出该空表示“除非”，因此应填unless引导条件状语从句。